<title>Variations in refractive index of color filter glasses</title>

Abstract

The focal shifts through optical bandpass filters built on color filter glass (CFG) substrates depend directly on substrate thickness and index of refraction. The filter set for HST Advanced Camera for Surveys (ACS) must have very tightly matched focal shifts. Knowing the index of refraction for substrate glasses allows precise substrate thicknesses to be specified. Unfortunately, index data is not provided by the glass manufacturer to the precision required to achieve adequately matched focal shifts. Therefore, the raw CFG materials for filters must be treated as unknowns with regard to refractive index. Using a simple yet accurate refractometer built at Goddard Space Flight Center (GSFC), indices of refraction were measured for approximately 30 different melts of CFG's from which ACS filters are made. In this paper, the process by which filters were made to have matched focal shifts is outlined, a compendium of all measured CFG index data is presented, and the variations of measured index from melt to melt of the same types of glasses are discussed and measured values compared to catalog values.